The Estuary as a Natural Water Treatment Plant for Microplastics

Microplastics (MPs) are becoming an important component of suspended particulate matter (SPM), especially in estuaries which are hotspots of MPs pollution. Most SPM in estuarine water are removed via flocculation and further deposited. Therefore, we hypothesise that there is an efficient removal process for MPs by flocculation, which are expected to decrease the overall load of MPs in the marine environment. Here we systematically studied and quantified the influence of MPs properties (size, shape, density, polymer type and weathering condition) on flocculation behavior with suspended sediment in estuary conditions. 

We chose over 20 types of MPs with different properties for 8 size ranges from (10-300 µm for fragments and microbeads, and 10-1000 µm in length for microfibers, respectively). The MPs with different properties and suspended sediment were flocculated in artificial seawater, and the MPs in the system were observed using fluorescence microscopy to distinguish the incorporated MPs and suspended MPs. The incorporation rate (IR) is the ratio of incorporated MPs to total MPs, which is the parameter to evaluate the interaction between MPs and flocs.

The IR decreased with increasing size for fragments and microbeads, and also decreased as the diameter of microfibers rose. The IR for fragments smaller 20 µm is extremely high, from 94.8% to 100%, but gradually decreased with increasing size. For fragments larger than 200 µm, the IR of Polyethylene (PE), Polypropylene (PP) and Polystyrene (PS) are lower than 20%, while higher than 50% for Polyethylene terephthalate (PET) and Polyvinyl chloride (PVC). The IR of PE microbeads is significantly lower than those of fragments. When the diameter of microfibers are smaller than 20 µm, the IRs are always higher than 90%, and the length has no effect on IR. While the IR of microfibers decreased with the length increasing when the diameter of microfibers is larger than 30 µm. According to extensive comparisons between 20 types of MPs, we found that the IR is normally higher in the small size, elongated, angular, high density, weathered, chemically active MPs, while lower in the large size, spherical, low density, pristine and chemically inactive MPs. The size plays the most important role, followed by shape.

There is an evidence that MPs are likely to be removed from the water column and hence estuary sediments are important sinks for MPs. This process reduces overall load to marine environments, but this is selective and depends on the characteristics of MPs. This study offers a reference to predict the preferential removal behavior of MPs in the estuary.